12.2.3.4 Climatic response to anthropogenic forcing

Figure 12.8: Simulated and observed zonal mean temperature change
as a function of latitude and height from Tett et al. (1996). The contour
interval is 0.1°C. All signals are defined to be the difference between
the 1986 to 1995 decadal mean and the 20 year 1961 to 1980 mean. (a), increases
in CO2 only (G); (b), as (a), but with a simple representation
of sulphate aerosols added (GS); (c), as (b), with observed changes in stratospheric
ozone (GSO); (d), observed changes.

We now consider the simulated response to anthropogenic
forcing. Models run with increases in greenhouse gases alone give a warming
which accelerates in the latter half of the century. When a simple representation
of aerosol effects is included (Mitchell et al., 1995b; Cubasch et al., 1996;
Haywood et al., 1997; Boer et al., 2000a,b) the rate of warming is reduced (see
also Chapter 8, Section 8.6.1).
The global mean response is similar when additional forcings due to ozone and
the indirect effect of sulphates are included. GCM simulations (Tett et al.,
1996; Hansen et al., 1997b) indicate that changes in stratospheric ozone observed
over the last two decades yield a global mean surface temperature cooling of
about 0.1 to 0.2°C. This may be too small to be distinguishable from the
model's internal variability and is also smaller than the warming effects
due to the changes in the well-mixed greenhouse gases over the same time period
(about 0.2 to 0.3°C). The lack of a statistically significant surface temperature
change is in contrast to the large ozone-induced cooling in the lower stratosphere
(WMO, 1999; Bengtsson et al. 1999).

The response of the vertical distribution of temperature to anthropogenic
forcing
Increases in greenhouse gases lead to a warming of the troposphere and a cooling
of the stratosphere due to CO2 (IPCC, 1996). Reductions in stratospheric
ozone lead to a further cooling, particularly in the stratosphere at high latitudes.
Anthropogenic sulphate aerosols cool the troposphere with little effect on the
stratosphere. When these three forcings are included in a climate model (e.g.,
Tett et al., 1996, 2000) albeit in a simplified way, the simulated changes show
tropospheric warming and stratospheric cooling, as observed and as expected
on physical principles (Figure 12.8). Note that this structure
is distinct from that expected from natural (internal and external) influences.

The response of surface temperature to anthropogenic forcing
The spatial pattern of the simulated surface temperature response to a steady
increase in greenhouse gases is well documented (e.g., Kattenberg et al., 1996;
Chapter 10). The warming is greater over land than over
ocean and generally small during the 20th century over the Southern Ocean and
northern North Atlantic where mixing extends to considerable depth. The warming
is amplified in high latitudes in winter by the recession of sea ice and snow,
and is close to zero over sea ice in summer.

Despite the qualitative consistency of these general features, there is considerable
variation from model to model. In Chapter 9, it was noted
that the spatial correlation between the transient response to increasing CO2
in different models in scenarios to the middle of the 21st century was typically
0.65. In contrast, the spatial correlation between the temperature response
to greenhouses gases only, and greenhouse gases and aerosols in the same model
was typically 0.85 (see Chapter 9, Table
9.2). Hence, attempts to detect separate greenhouse gas and aerosol patterns
in different models may not give consistent results (see Section
12.4.3.2).